Field of the Invention
The present invention relates to multispectral imaging using optical elements capable of simultaneously focusing light from one or more spectral bands to a common focal plane and a detector capable of capturing the multispectral image.
Description of the Prior Art
Imaging in several spectral bands is used for surveillance and reconnaissance. Some of the examples include imaging in day and night by soldiers for situational awareness on a battlefield, aerial reconnaissance or surveillance over land for border protection, at sea for platform protection, or for property protection. Each spectral band provides unique information depending on the weather conditions and the intensity of solar light. The visible imagers operate between the wavelengths (λ) of about 0.4 μm to 0.7 μm and provide very high-resolution imagery in clear daylight; however, the imagery is very limited on a hazy day with significantly reduced resolution and range. A shortwave infrared (SWIR) imager operating in the wavelength of about 0.9 μm to about 2 μm has very good haze penetration and can see clearly when the visible camera fails to produce any meaningful information. A midwave infrared (MWIR) thermal imager operating in about 3 μm to about 5 μm wavelength can be used in conjunction with the visible imager for hot object discrimination. For example, a visible imager cannot tell which car in a parking lot or which boat on a pier is running, while a MWIR imager can discriminate the hot engine of a running car or boat from nearby cold engines. A longwave infrared (LWIR) imager operating in the wavelength range of 8 μm-12 μm or 8 μm-14 μm is also used to produce images at day and night. It can image through smoke when other imagers cannot. It is also sensitive to very small temperature differences between an object and its surroundings for target identification in the dark.
Often, imaging in a single band is not enough for positive target identification for the reasons described above. Multispectral imagers based on reflective optics are available. However, these imagers are typically bulky and have a very narrow field of view. They also run into central obscuration due to a secondary mirror. Applications such as wide area surveillance require very wide field of view that can only be achieved through refractive optics. Currently, multispectral imaging using refractive optics is performed with separate cameras with separate apertures, optical trains, and sensors for individual spectral bands. The imaging data obtained can be analyzed separately but is often combined or stitched into a composite multispectral image. Compact, lightweight multispectral imagers using a common aperture would provide practical advantages. Recent developments in multispectral detector technology (Rogalski, “Infrared Detectors for the Future,” Acta. Phys. Pol. A, 116(3), 389-406 (2009); Reibel et al., “Infrared Dual Band detectors for next generation,” Proc. of SPIE 8012, 801238 (2011); and Dixon et al., “Dual-Band Technology on Indium Gallium Arsenide Focal Plane Arrays,” Proc. of SPIE 8012, 80121V (2011)) and common focal plane arrays (FPA) can potentially enable compact imagers but are limited by developments in multispectral refractive optics. Currently there are very few optical materials commercially available that cover the transmission range from SWIR to LWIR. These materials also cover a very narrow region in the map of refractive index and dispersion. Thus, several optical elements are needed to correct for chromatic aberrations over a broad wavelength spectrum (U.S. Pat. No. 7,369,303 to Tejada (2008) and Sparrold et al., “Refractive Lens Design for Simultaneous SWIR and LWIR Imaging,” SPIE Proceedings Vol. 8012, 801224 (2011)) adding size and excessive weight to the imaging system. For example, an F/1, 50 mm SWIR/LWIR imager described by Sparrold et. al (Sparrold et al., “Refractive Lens Design for Simultaneous SWIR and LWIR Imaging,” SPIE Proceedings Vol. 8012, 801224 (2011)) required eleven optical elements for correction of chromatic aberrations over both of the wavelength bands (0.9-1.7 μm and 8-10 μm) and correction of other optical aberrations to meet the required performance specification (23° full field of view, 30% contrast at 20 line pairs per mm). Such an imager will be very heavy and it will be challenging to meet tolerances due to the alignment of all eleven elements and optical distortion in a useful temperature range of −50° C. to 50° C. due to the difference in the thermo-optic coefficient (dn/dT) of the individual elements. In addition, there are 22 optical surfaces requiring broadband AR coatings.
There have been several dual-band optical designs and multispectral imagers based on refractive optics in SWIR+MWIR or MWIR+LWIR or SWIR+LWIR imagers in recent years, but all use optics from a handful of materials which have a broad spectral transmission covering all the wavelengths of interest (U.S. Pat. No. 7,369,303 to Tejada (2008); Sparrold et al., “Refractive Lens Design for Simultaneous SWIR and LWIR Imaging,” SPIE Proceedings Vol. 8012, 801224 (2011); and Palmer et al., “SOMEWHERE UNDER THE RAINBOW: The Visible to Far Infrared Imaging Lens”, SPIE Proceedings Vol. 8012, 801223 (2011)). The limited number of optical materials to choose from leads to complex lens systems with either poor imaging performance or excessive size and weight due to the use of a large number of optical elements needed to meet the specifications of required image quality. Use of a large number of optical elements also makes the alignment tolerances very challenging. Another drawback of using an imaging system with a large number of optical elements is the large number of air-optic interfaces resulting in Fresnel reflection loss per surface (R) given by the relation:R=[(n−1)/(n+1)]2 where n is the refractive index of the optic at a given wavelength. These Fresnel reflection losses multiply with increasing number of air/optic interfaces from the increased number of optical elements and reduce the overall brightness of the image. An imager that produces comparable imaging performance with fewer optical elements and fewer air/optic interfaces will have smaller size, lower weight and will produce an image with higher brightness.
An imaging system's performance is measured and reported in terms of its Modulation Transfer Function (MTF), which describes the contrast of the image relative to that of the object and is plotted as a function of resolution (spatial frequency) in line-pairs per millimeter (lp/mm). Diffraction fundamentally limits the maximum attainable MTF of an imaging system. Imagers with resolutions as good as this theoretical limit are not uncommon and are said to be diffraction limited.
One challenge in designing and building a multispectral imager with reduced size, weight and improved performance, is reducing the weight of the imaging optics, since the focal plane array (FPA) sensors are becoming more capable and more compact. This includes reducing the number of optical elements and number of air/optic interfaces while meeting the imaging system performance requirements. Clearly, there is a need to increase the number of available broadband multispectral optical materials that can be used in lightweight and compact multispectral imaging solutions. The “glass map” is relied on for optical material selection while designing optics. A glass map displays various optical materials (including single crystals, polycrystalline ceramics and amorphous glasses) on a refractive index versus Abbe number plot. The Abbe number in the IR region is defined as:
            Abb      ⁢      é        ⁢                  ⁢    Number    ,      v    =                            n          center                -        1                              n          short                -                  n          long                    
where nshort and nlong are the indices at the extreme ends of the wavelength range and ncenter is the index at center wavelength (e.g. for LWIR nshort=index at 8 μm, nlong=index at 12 μm, and ncenter=index at 10 μm). The short and long wavelengths may vary depending on the application, FPA sensor, and design criteria. A low Abbe number represents high dispersion. FIG. 1 shows a glass map (Abbe diagram) for visible glasses only. The map shows a large selection of materials for optics design in the visible region. When designing achromatic doublets to do color correction over a wider wavelength range, optics design engineers use the first optical element (crown) with positive power and high index but low dispersion for focusing power with minimal chromatic aberration. The second element (flint) with negative power is chosen to have low refractive index but a high dispersion to correct for chromatic aberration with minimal reduction in focusing power. The glass map narrows in SWIR with not as many possibilities as in the visible. However, the choice becomes very limited for MWIR and LWIR with only a handful of optical materials.